This invention relates to a system for the separation of air by two-stage low-temperature rectification wherein the air is subjected to a preliminary purification step, compressed, and cooled by heat exchange with separation products.
In such processes, the oxygen product of the low pressure stage is generally evaporated in indirect heat exchange contact with condensing nitrogen of the high pressure stage. The relationship of the pressures of the two rectification stages is based on the requirement that the condensation temperature of the nitrogen must be somewhat above the vaporization temperature of the oxygen. Due to this thermodynamic correlation between the pressure conditions in both stages, the pressure of the high pressure stage is clearly dependent on the desired pressure of the products withdrawn from the low pressure stage. Consequently, if it is desired to obtain the separation products at higher pressures, the pressure of the high pressure stage must also be raised, and, therefore, the entire air feed must be compressed to a higher pressure. This results in high operating costs, particularly in the case of large-scale plants. For details of this conventional air separation process, attention is invited to, for example, U.S. Pat. No. 3,070,966 (M. Ruhemann and L. Putman) and U.S. Pat. No. 3,447,331 (K. Smith, BOC).